We were talking about the scene between Auron and Tidus on the docks when Auron tells T. the truth about his father, T. loses it and blames everything on Auron, and Auron starts laughing.

I noted that Auron has a certain sympathy for Tidus precisely because Auron himself used to be very high-strung. The only ways he knows to deal with grief are to get angry (which he used to do), weep (which is what Tidus does) or laugh (which is what Auron does now).